Nuclear industry equipment and structural materials are subjected to radioactive contamination during use. The equipment, which is usually constructed from stainless steel or other metal, must therefore be routinely cleaned or otherwise treated to render it safe for further use or disposal. Such treatments include scrubbing, washing or abrading of the surface of the material in an effort to remove the deposits.
One such decontamination technology involves immersing the contaminated material in a carbonate solution and subjecting it to electrolysis sufficient to cause stripping or separating of the contaminants from the surface of the metal. Representative examples include the following U.S. patents; U.S. Pat. No. 3,873,362 to Mihram et al., U.S. Pat. No. 4,217,192 to Lerch et al., U.S. Pat. No. 4,537,666 to Murray et al., U.S. Pat. No. 4,663,085 to Edna et al., U.S. Pat. No. 5,102,511 to Suwa et al., U.S. Pat. No. 5,322,644 to Dunn et al. and U.S. Pat. No. 5,340,505 to Hanulik et al.
In each of the above cases, the chemical decontaminate comprises various acidic and/or alkaline reagents, with or without oxidizing agents. In these chemical-only decontamination methods, the electrolytic cell is employed solely for the purpose of regenerating the decontamination reagent. Accordingly, these methods do not teach use of the electrolytic cell as the primary mechanism for decontamination. Further, the reagents are in and of themselves hazardous or dangerous, particularly the acidic reagents. Finally, prior art acidic or alkaline reagents are destructive to the metal equipment being treated since they invariably erode or etch the surface of the metal during treatment.
Other prior art decontamination methods include the following:
U.S. Pat. No. 4,193,853 to Childs et al. teaches a metal decontamination system employing an electrolyte comprising nitrate salts, borate, fluoride or oxalate individually and at a basic pH. In a preferred embodiment, the electrolyte contains a combination of nitrate, borate, fluoride and oxalate ions and a pH between 7 and 11.
U.S. Pat. No. 4,481,089 to Izumida et al. teaches a metal decontamination system employing a neutral salt electrolyte with an alternating electrolysis method. Contamination is removed by applying a current to the electrochemical cell and at programmed intervals the current is reversed thereby causing loosening or shaking off of the contaminant from the surface of the metal. As is apparent, programmed current fluctuations requires the system be equipped with appropriate controls which adds to the cost of the system.
U.S. Pat. No. 4,481,090 to Childs teaches an improved system wherein a more efficient acidic electrolyte, which may include high concentrations of nitrate, is substituted for a prior art alkaline nitrate, borate, fluoride and oxalate electrolyte.
U.S. Pat. No. 4,615,776 to Sasaki et al. discloses an electrochemical metal decontamination method having a highly concentrated phosphoric acid solution for the electrolyte. The pH of the electrochemical cell is approximately 2.
U.S. Pat. No. 5,439,562 to Snyder et al. discloses a nickel recovery process employing electrochemical metal decontamination and in particular method a nickel recovery process directed toward removal of actinide radionuclides and technetium.
U.S. Pat. No. 5,614,077 to Wittle et al. discloses an electrochemical decontamination system including a reaction chamber where the pH and electrical current may be varied as required to precipitate out any radionuclide contamination. There is no disclosure of a specific electrolyte nor is contact provided between the anode or cathode and the item being decontaminated.
In view of the above, a need has existed in the art for a decontamination method that will not corrode or otherwise damage the metal equipment being treated thereby allowing the decontaminated equipment to be reused.